The general objective of this study is to elucidate the mechanisms regulating progesterone production in primate pregnancy. Using the pregnant baboon as a model the following aspects are under investigation: 1) determination of progesterone serum concentrations; 2) measurement of metabolic clearance rates and calculation of blood production rates of progesterone; 3) evaluation of the relative importance of serum protein binding and metabolism in determining clearance; 4) estimation of the contributions of placenta and ovary to total production; 5) examination of circulating precursors and factors controlling their conversion to progesterone.